Designing reinforced concrete beams for shear involves a crucial aspect – determining the spacing between stirrups. Shear forces act perpendicular to the longitudinal axis of the beam. Typically, these forces are highest at beam supports and diminish towards the center of the span.

Reinforced concrete beams often experience cracks in flexure due to principal tension stresses. These stresses change from horizontal at the longitudinal reinforcement to 45 degrees at the neutral axis and vertical at the point of maximum compression stress.

Shear reinforcement, in the form of stirrups, plays a vital role in preventing and resisting shear cracks. Shear design considers flexural failure, diagonal tension failure, and shear compression failure as potential modes of failure.

**Modes of Failure**

**Flexural Failure :**Vertical cracks mainly in the middle third of the span.**Diagonal Tension Failure :**Beam strength in diagonal tension is lower than in flexure.**Shear Compression Failure :**Occurs in beams with small shear span/depth ratios.

Stirrups in shear reinforcement perform four essential functions:

- Carrying a portion of the flexural factored shear force.
- Restricting the growth of diagonal cracks.
- Holding longitudinal reinforcement bars in place.
- Providing some confinement to concrete in the compression zone if stirrups are closed ties.

The critical section for shear design depends on beam support conditions. ACI Code provisions guide shear strength calculations using the formula Vu = Vn, where Vn is the sum of concrete and stirrup contributions.

Shear strength (Vn) combines the strengths of concrete (Vc) and stirrups (Vs). For circular sections, ACI specifies area calculation using the product of diameter and effective depth.

**Minimum Shear Reinforcement :**Web reinforcement may not be needed if Vu ≤ (∅Vc). However, minimum reinforcement is required per ACI Code.**Minimum and Maximum Stirrup Spacing :**Avoid spacing stirrups closer than 100 mm. Determine maximum spacing based on ACI Code provisions.

The shear design involves various steps, including computing ultimate shear force, estimating concrete shear strength, and selecting appropriate stirrup spacing.

Consider a simply supported beam with a 400 mm width and 550 mm effective depth, subjected to a total factored load of 137 KN/m over a 6 m clear span. The beam is reinforced with tensile steel, and shear design using vertical U stirrups is illustrated.

**Shear Force Calculation :**Using shear force diagrams.**Design Shear Strength of Concrete :**Computed using ACI Code provisions.**Shear Reinforcement Design :**Determining stirrup diameter, spacing, and distribution.

**In conclusion,** designing reinforced concrete beams for shear involves a systematic approach considering various factors to ensure structural integrity and safety.